U.S. President Barack Obama has indicated that the United States and its allies will not back down in pressuring Russia over the annexation of Crimea.

Obama said on March 25, the United States is concerned about the massing of Russian troops along Ukraine's border but he added Russia seems to be "making a series of calculations" as the Kremlin takes note of international opposition to Russian moves in Crimea.

Obama said it would be a bad choice for Russian President Vladimir Putin to make further moves into Ukraine and repeated that there is another path available to Russia.

Speaking at a news conference with Dutch Prime Minister Mark Rutte, Obama called Russia a "regional power" that is threatening a neighbor out of weakness not strength, noting if Russia had substantial influence in Ukraine it would not need to send troops there.

Obama conceded however that Russia has military control over Crimea but reiterated the U.S. and its allies are "not recognizing what is happening in Crimea."

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He dismissed the referendum held in Crimea on March 17 to join Russia as "sloppily organized over the course of two weeks" and rejected that the vote could "somehow be a valid process."

But Obama cautioned that "it would be dishonest to suggest there is a simple solution to what has already taken place in Crimea."

Asked about the possibility of a military conflict erupting over Ukraine, Prime Minister Rutte said he did not think that was likely.

Obama added Russia is not the number one U.S. national security threat and that he is more concerned about the possibility of a nuclear weapon going off in Manhattan.

Rutte said the West stood ready to impose more sanctions on Russia but he warned Moscow's response could hurt the economies of Europe, the United States, Japan, and other countries.

Rutte said, "Obviously, we will make sure that we will design these sanctions in such a way that they will have maximum impact on the Russian economy and not on the European, the Canadian, the Japanese, or the American economy."